It is proposed to continue a research program designed to provide information about DNA base damage and its effects. The quantities and types of damage to thymine and adenine bases produced by ionizing radiation in vivo will be determined in bacterial strains selected for their known radiation sensitivities and for their genetically characterized ability to repair radiation damage. Specific products of base damage such as thymine glycols will be measured by HPLC as a function of radiaton dose and time after irradiation during the period in which radiation damage is repaired. Modifications in quantity and types of DNA damage induced by the presence of spin-labeled platinum drug molecules during irradiation will continue to be studied and will be extended to Mammalian Cells in culture. Studies on the mechanisms of radiation damage to 5-bromouracil bases which are substituted for thymine base in the DNA of E. Coli cells in vivo will be completed. Radiation effects on carbon-13 enriched DNA in vivo and in vitro will be studied by 13 C-NMR techniques.